1. Field of the Invention
The present invention relates to an austenitic stainless steel, especially for making wire, having an inclusion cleanliness for use in the field of drawing wire down to diameters of less than 0.3 mm and in the field of producing components subjected to fatigue.
2. Discussion of the Background
Iron alloys containing at least 10.5% chromium are referred to as stainless steels. Other elements form part of the composition of the steels so as to modify their structure and their properties.
Austenitic stainless steels have a defined composition. The austenitic structure forms after transformation by a heat treatment of the rapid cooling type.
From a metallurgical standpoint, it is known that certain alloying elements forming part of the composition of the steels favor the appearance of the ferrite phase, which has a metallographic structure of the body-centered cubic type. These elements are called alphagenic elements. Among these are chromium, molybdenum and silicon.
Other so-called gammagenic elements favor the appearance of the austenite phase, having a metallographic structure of the face-centered cubic type. Among these elements are carbon, nitrogen, manganese, copper and nickel.
In the field of wire drawing, for example, it is known that, in order to obtain a wire having a diameter of less than 0.3 mm, called fine wire, the stainless steel used must not have inclusions whose size causes the wire to break during drawing.
In the production of austenitic stainless steels, as for all other steels produced using conventional means economically suitable for mass production, the presence of inclusions of the sulfide or oxide type is routine and irremediable. This is because stainless steels, in the liquid state, may, because of the production processes, have oxygen and sulfur contents in solution of less than 1000 ppm. When cooling the steel in the liquid or solid state, the solubility of the oxygen and sulfur elements decreases and the energy of formation of oxides or sulfides is reached. Inclusions therefore appear, these being formed, on the one hand, from compounds of the oxide type containing oxygen atoms and alloying elements eager to react with oxygen, such as calcium, magnesium, aluminum, silicon, manganese and chromium, and, on the other hand, compounds of the sulfide type containing, sulfur atoms and alloying elements eager to react with sulfur, such as manganese, chromium, calcium and magnesium. Inclusions may also appear which are mixed compounds of the oxysulfide type.
The amount of oxygen contained in the stainless steel may be reduced by using powerful reducing agents, such as magnesium, aluminum, calcium, titanium or a combination of several of these, but these reducing agents all lead to the formation of inclusions rich in MgO, Al.sub.2 O.sub.3, CaO or TiO.sub.2, which are all in the form of crystallized refractories that are hard and cannot be deformed under the conditions of rolling the stainless steel. The presence of these inclusions causes problems, for example breakages in wire drawing, and fatigue fractures in products produced from the stainless steel.
Patent Application F 95 04 782 discloses the treatment of an austenitic stainless steel for the production of wire which can he used in the wire-drawing field and in the field of producing components subjected to fatigue.
It has been observed in general, depending on the various compositions, that the stainless steel described does not perform reliably both from the standpoint of the number of breakages during wire drawing and from the standpoint of fatigue behavior. In other words, the steel compositions described in the patent application of the prior art are not entirely satisfactory, especially because the inclusion field is defined much too broadly.